tie rod repositioning

[fireslave]

I have a 240sx project car that I am swapping a v6 into, and i was forced to relocate the steering rack because of clearance issues. I had to lower it 2 inches because the pinion housing was in the way of the starter motor.

Anyways, I figured that I could just add a 2 inch drop to the outside of the tie rod as well, and everything would even out.

I neglected to realize that the arm on the spindle that the tie rod connects to travels in an arc that is not completely parallell to the ground. Its actually a strut front suspension, so its rather far from it.

I modled a sloppy non-dimensionally accurate version in 3d, and it appears that it increases the ackermann, especially as it gets closer and closer to lock. However, i dont know how much exactly.

Does anyone forsee any major problems because of this? I was thinking of moving the tie rod pick up points slightly to compensate for the extra akermann (steering rack is behind the ball joints as usual)

Does anyone know of a program that is free (or very cheap) that is designed specifically for calculating strut front suspensions and steering? I have a free trial of alibre, but its not giving me any numbers, im just going by eye.

thanks!
-Allen

 

[fireslave]

To clarify, I will be lowering the outer tie rod pick-up points, otherwise bump steer will be horrible.

I am also considering moving the pick-up points outward as well, to reduce akermann back to a normal level.

excessive akermann shouldn't hurt handling a lot, but it causes the inside front tire to overheat on high speed corners because it scrubs too much...

 

[GregLocock]

Some people use that effect of Ackerman to improve rotation on corner entry, so I wouldn't panic.

You can get what you want using a 2d drawing board, so there is no real reason why alibre can't do what you want.

Anyway see the FAQ for the suspension forum, it has a list of programs, from free through to ridiculous. In your case if you can afford 300 bucks get susprog, as wishbone doesn't do mac struts (if you are really just interested in ackerman vs steer that doesn't matter). In fact the demo version of susprog may be all you need.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[fireslave]

i didnt realize this until i modeled it in 3d, but because of the arc the knuckle swings, it effectively changes the length of the steering arm as it swings.

in my attached pic, you can see that the tie rod is at a larger angle than it would be if it were stock (stock is represented by the red line) causing the inside tire to steer extra sharp. the rack is a couple inches ahead of the outer tie rod ends , so the outside tire is less affected by this.

Of course, i think it exaggerated the effect. Stock when the wheels are straight, the outer tie rod is at about the same height as the ball joint.

 

[fireslave]

heres another pic.

I am glad you don't think it will be a big problem though, thanks kinda what I thought and hoped.

heres another pic... bumpsteer is fine, but it does add a little akermann.

this is why i didnt want to try and do it in 2d.




I am thinking the thing to do might be to make the thing in two pieces, so I can just replace the lower half to adjust the outer hinge point if it presents a problem. the top half has to be welded on... 2 inches is too much to trust to an extended stud IMO. especially since when i lower the car I might have to extend it even a little more...



I suspect the symptoms of too much akermann would be:
-inside tire wear and noise coming from inside tire
-inside tire locks up easier under braking
but maybe not... anyone know what to watch out for?

 

[fireslave]

ah yes, the camber change (especially on the inside wheel) is also responsible for the amplification of the akermann effect.

never even thought of that until i tried it in 3d.


my example pic is prob. way more than 2 inches though... (i don't have exact dimensions to build a good model with and the cars not available to me at this exact moment)

 

[GregLocock]

Have a play in susprog's demo. I think you are on the right track.

You haven't said what you want to use the car for, that makes a difference.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[fireslave]

the plan is street/auto-x/open track days

actually, maybe just street/open track. i dont think it will be too competetive in auto-x because the engine swap and steering system mods will probably bump it into an overly competetive class (suspension mounting points cannot be changed in most of the lower classes... i dont know if this qualifies or not). but who knows, maybe it'll be faster than i expect.

no wheel-to-wheel racing either. i cant afford to be denting this thing too much.

actually, i kinda think that most of the turns I take at 10/10ths will be taken at lower speeds anyways, just because of the nature of the types of driving i'll be doing.... there arent any 100mph sweepers in auto-x, and taking a turn at 10/10ths is not a wise idea on the streets anyways.


ive heard about people adjusting their akermann before... i wonder how much of an adjustment you have to make before it causes a noticeable difference in handling... would 1/8th of an inch in/out over a 6 inch arm be noticeable? how about 1/4? street tires have a fairly decent amount of slip angle before they break loose anways, even performance summer tires.